Polymer blends which have a combination of both thermoplastic and elastic properties are generally obtained by combining a thermoplastic resin with a rubbery composition in a way such that the rubber component is intimately and uniformly dispersed as a discrete particulate phase within a continuous phase of the thermoplastic. This is usually achieved by exposing the blend of the thermoplastic component and the rubber component to conditions of high shear and temperature while at the same time vulcanizing the particulate phase of the rubber component, for instance, in an extruder or a Brabender mixer. This process is commonly referred to in the art as dynamic vulcanization.
In the prior art thermoplastic vulcanizates (‘TPV’) comprise polypropylene as the thermoplastic component and as the rubbery phase a rubber such as natural rubber, elastomeric copolymers of ethylene and propylene such as ethylene/propylene rubber (EPR) and ethylene/propylene/(non-conjugated diene) monomer rubber (EP(D)M), butyl rubber, halobutyl rubber, halogenated (e.g. brominated) copolymers of p-alkylstyrene and an isomonoolefin, homo- or copolymers from at least one conjugated diene, or combinations thereof. Among these rubbers, EPDM as the elastomeric component has gained particular attraction and is most commonly used as the elastomeric compound in thermoplastic vulcanizates.
Thermoplastic vulcanizates from polypropylene and a rubber have gained wide acceptance as a substitute for thermoset rubbers in a large variety of applications. The reason is their unique characteristic that, notwithstanding the fact that the rubber component is partially or fully cured, the compositions can be processed and reprocessed by conventional plastic processing techniques such as extrusion, injection molding, blow molding, and compression molding. Scrap or flashing can be salvaged and reprocessed.
It would be desirable for many of these applications to increase the softness of the material or, conversely, to decrease hardness. Within certain limits, this can be achieved by either increasing the amount of extender oils as additives relative to the total amount of thermoplastic resin and rubber or by increasing the amounts of rubber relative to the amount of thermoplastic resin. However, if a certain ratio of the amount of extender oil to the total amount of thermoplastic resin and rubber is exceeded, the extender oil can not reliably be retained within the thermoplastic vulcanizate. Thus, under certain conditions of use (such as elevated temperature, mechanical stress) the extender oil tend to leak from the composition, comparable to leaking of a super-soaked sponge. This effect known as ‘oil weeping’ not only affects the appearance of the composition but also impacts its mechanical properties since the additional amount of extender oil which was added to the composition in order to soften it is lost. On the other hand, increasing the relative amount of the rubber might provoke a reversal of morphology, i.e., the rubber phase becomes the continuous phase while the thermoplastic resin is dispersed therein. A composition having such a reversed morphology, however, is no longer processable and reprocessable by conventional plastic processing techniques. Furthermore, a large decrease in hardness cannot be obtained by either of these two approaches.
Nevertheless, a need exists in many fields of applications for polymer compostions that are soft and processable by conventional plastic processing techniques (i.e. thermoplastic vulcanizates). It goes without saying that for the sake of wide and universal applicability a decrease of hardness should not result in an unacceptable loss of other important mechanical properties such as ultimate tensile strength, elongation at break, tear, compression set, and the like.
Published international patent application WO 99/27001 discloses a thermoplastic vulcanizate composition comprising from about 20 to about 85 parts by weight rubber and from about 15 to about 80 parts by weight of semi-crystalline polypropylene wherein said parts by weight are based upon 100 parts by weight of said rubber and said polypropylene, and a thermoplastic random ethylene copolymer, wherein the weight ratio of said polypropylene to said random ethylene copolymer is from about 100:5 to 100:150 and wherein said random ethylene copolymer comprises from about 70 to about 95 weight percent repeat units from ethylene and from about 5 to about 30 weight percent of repeat units from one or more other ethylenically unsaturated monomers based upon the weight of said random ethylene copolymer. Thus, the thermoplastic vulcanizate composition comprises between about 12 and about 77 weight percent of polypropylene, based on the total amount of polypropylene, random ethylene copolymer and rubber. The thermoplastic random copolymer of ethylene is prepared by metallocene catalysis. The aim of that disclosure is to provide thermoplastic vulcanizates having increased elongation at break and an increased total toughness. Increasing the softness of the thermoplastic vulcanizate is not addressed. In contrast, the thermoplastic vulcanizates according to the present invention are intended to have improved softness without deterioration of the mechanical properties.
It has been an object of the present invention to provide thermoplastic vulcanizate compositions having superior softness while retaining typical properties known for thermoplastic vulcanizate compositions.
Another object of the invention has been to provide a process for preparing said thermoplastic vulcanizate compositions.
A further object of the invention has been to provide articles comprising said thermoplastic vulcanizate compositions.